Publication Type:Journal Article
Source:Economic GeologyEconomic Geology, Volume 112, Number 3, p.629-660 (2017)
The 007 gold deposit is located within the 2.99–2.70 Ga Rice Lake greenstone belt, Manitoba, Canada. Aurifer- ous quartz-ankerite veins (± albite ± sericite ± chlorite) contain early, coarse-grained, barren quartz (Q1) and secondary, finer grained quartz (Q2). The latter occurs as polycrystalline “ribbons” and microveinlets hosted along Q1 grain boundaries that formed by dynamic recrystallization of Q1, and later dissolution-reprecipitation, respectively. Deposition of electrum (65–86 at. % Au) postdated Q1 formation and was broadly syngenetic with Q2 formation. Electrum occurs in two forms: (1) along sutured (healed, recrystallized) and partially sutured grain boundaries representing dissolution-reprecipitation surfaces between adjacent Q2 grains in association with coeval sericite and ankerite, and (2) as inclusions coeval with Au-Ag-Ni-Pb tellurides hosted within pyrite grains coeval with Q2 and secondary ankerite and sericite. Pyrite trace element mapping (by LA-ICP-MS) dis- plays complex metal zonation indicating significant variations in the availability of metals during various stages of pyrite formation. Mapping of trace elements in pyrite shows that there were two Au precipitation events, one coeval with Zn, In, Sn, Cd, Ag, and Te enrichment in the cores of pyrite and another fracture-controlled stage coeval with Ag, As, Se, Pb, Bi, Ni, and Co enrichment.<br/>Four distinct types of fluid inclusions are identified in quartz: (1) type 1 aqueous-carbonic inclusions<br/>(H2O-NaCl-CO2 ± CH4 ± N2) hosted throughout Q1 quartz with a range in the melting point of CO2 (TmCO2) from –59.6° to –56.6°C due to minor and variable CH4 (≤1.4 mol %) and N2 (≤1.7 mol %) in the carbonic phase,<br/>salinities from 4.1 to 9.7 wt % NaCl equiv, and total homogenization (ThTOT) between 278° and 346°C to the liquid phase; (2) type 2 carbonic inclusions with a minor aqueous component (CO2 ± CH4 ± N2 ± H2O-NaCl)<br/>hosted along Q1-Q2, and Q2-Q2 grain boundaries; TmCO2 in these inclusions ranges from –58.0° to –56.6°C due to minor CH4 and N2 in the carbonic phase; (3) type 3 aqueous inclusions with a minor carbonic component (H2O-NaCl ± CO2 ± CH4 ± N2) that are hosted along Q1-Q2 and Q2-Q2 grain boundaries; these inclusions show a range in salinity (4.0–12.4 wt % NaCl equiv) similar to type 1 inclusions and have ThTOT between 273° and 315°C; and (4) type 4 aqueous inclusions (H2O-NaCl) that occur in secondary trails crosscutting (i.e., postdating) Q1 and Q2 domains and grain boundaries; these inclusions are more saline (14.8–17.5 wt % NaCl equiv) than type 1 and 3 inclusions. Importantly, whereas type 1 inclusions were trapped during the growth of Q1 (prior to gold deposition), the common occurrence of texturally coeval type 2 and 3 inclusions and electrum along Q1-Q2 and Q2-Q2 grain boundaries suggests that these fluids are likely related to gold deposition. <br/>In situ oxygen isotope analysis of barren Q1 and auriferous Q2 shows large inter- and intra-grain variations in composition. These variations in δ18O and the corresponding temperature-dependent δ18OH2O values for the fluids that equilibrated with Q1 (–3.7 to +6.8‰) and Q2 (–3.3 to +8.4‰) are not the consequence of fluid mixing. Rather, grain-scale δ18O variation may have involved Rayleigh fractionation within the veins if they are considered to have acted as a partially closed system rather than a continuously open and replenished flow-through system. <br/>An approximate P-T window of gold deposition is constrained using fluid inclusion microthermometry fortype 2 and 3 inclusions (trapped synchronous to gold precipitation), combined with the relevant stability bound- aries for Au-Ag-Te phases that occur as inclusions in texturally coeval pyrite. The maximum conditions for gold precipitation are ~1 kbar and 320°C. Differences between minimum Ptrapping and Ttrapping for pre-gold fluids (type 1 inclusions) and maximum Ptrapping and Ttrapping for syn-gold fluids (types 2 and 3) suggest that immiscibil- ity was responsible for gold deposition and occurred due to significant fluctuations in pressure (>1 kbar) but with neglegible cooling from the barren Q1 to the auriferous Q2 stage.